Apologies for not recognizing that we'd spoken before on this issue. I can confirm that the evenly rule converts the base spline to curves, I'll report this issue and see if anything can be done to change this behavior in future releases. In the meantime I think your other instinct is correct and material IDs are likely to be the best way to resolve this.
I realize there are some problems with start and end segments however you may be able to resolve this by breaking the spline where you'd like the Start/End segments to appear. You can also use conditional operators to change the start and end segments based on material ID if necessary.
In the attached example I've used an ID of 10 for the curves, and an ID of 1 and 2 to show the idea of two variations of fence. The spline has been broken between these 2 IDs to force start and end segments.
Hopefully this will help with your situation, let me know if you need any more help.